LinuxKPI: add the "dummy" includes directory to builds

While we could add the dummy includes directory manually to only the
drivers needing it, it seems a lot easier to simply add it to all
without any expected harm.

This is needed for more drivers (and to remove some #ifdef in current
ones) with empty header files being present not yielding errors.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Reviewed by:	hselasky, imp
Differential Revision: https://reviews.freebsd.org/D36684

sys/conf/kern.pre.mk

@@ -293,7 +293,8 @@ NORMAL_CTFCONVERT=	@:
 .endif
 
 # Linux Kernel Programming Interface C-flags
-LINUXKPI_INCLUDES=	-I$S/compat/linuxkpi/common/include
+LINUXKPI_INCLUDES=	-I$S/compat/linuxkpi/common/include \
+			-I$S/compat/linuxkpi/dummy/include
 LINUXKPI_C=		${NORMAL_C} ${LINUXKPI_INCLUDES}
 
 # Infiniband C flags.  Correct include paths and omit errors that linux

sys/conf/kmod.mk

@@ -109,7 +109,8 @@ LINUXKPI_GENSRCS+= \
 	opt_stack.h
 
 LINUXKPI_INCLUDES+= \
-	-I${SYSDIR}/compat/linuxkpi/common/include
+	-I${SYSDIR}/compat/linuxkpi/common/include \
+	-I${SYSDIR}/compat/linuxkpi/dummy/include
 
 CFLAGS+=	${WERROR}
 CFLAGS+=	-D_KERNEL